Advice on selecting the right size electric outrunner motor - RC Groups
Shop our Airplanes Products Drone Products Sales
Thread Tools
Jan 01, 2011, 08:55 AM
Registered User

Advice on selecting the right size electric outrunner motor

Is their information available to cross reference electric outrunner electric motors to glow engines? I am trying to determine what size electric outrunner motor to used in converting glow airplanes in the ol' hangar... Look forward to your feedback and guidance.
Sign up now
to remove ads between posts
Jan 01, 2011, 09:17 AM
Registered User
Ken Myers's Avatar
There is no cross-reference. The two types of power are completely different.

Rule of thumb: engine displacement times 1500 watts in to 2000 watts in for 2-stroke glow.

.45 2-stroke * 1500 about 675 watts in
.45 2-stroke * 2000 about 900 watts in

Watts in are measured by a power meter/wattmeter/whattmeter

In general, e-power uses the largest diameter possible for the given airframe.

Others will chime in with lots more, I'm sure. It also depends on the "mission" of the plane and the way you like to fly a given aircraft.

The section called Glow to Electric Conversions has lots of successful conversions. Search that topic area and you'll probably find some of the planes in your hangar.
Last edited by Ken Myers; Jan 01, 2011 at 09:24 AM.
Jan 01, 2011, 09:22 AM
Registered User
aim for about 100+ watts-in per lb for sport flying and 150+ for more extreme aerobatics
Jan 01, 2011, 09:53 AM
Registered User
Dr Kiwi's Avatar
As Ken says... one fairly useful rule of thumb (Lucien Miller of Scorpion fame advocates this one) is "Watts = cubic inches x 2000".
Jan 01, 2011, 11:38 AM
Registered User
335driver's Avatar
So, if I have a 6 lb twin I would need 2-300W motors? Is that the rated W of the motor?
Plans call for 2-.30 size IC engines. Weight now w/o paint is 4lbs, so I'm figuring 6 w/paint and servos, motors should I add another 2lbs for batteries? That would up it to 8 lbs and 400W per motor. What am I missing?
Jan 01, 2011, 12:23 PM
Registered User
Dr Kiwi's Avatar
Using the rule of thumb, .30 IC equates with 600W... that would mean 1200W total = 150W/lb for your 8lb plane. Now you can probably get away with a bit less, but I wouldn't go much below 800W total (100W/lb)... given the over-optimism of motor specs I'd choose two motors said to be good for 600W each... then run 'em at 400W each.

First thing to do is determine the best prop size (usually as large diameter as will give you ground clearance!)... then the cell count/Kv to spin such props at 400W. High voltage/low amperage is usually better than the reverse.
Jan 01, 2011, 12:54 PM
Suspended Account
I believe MPI ( has some good info. Here this is a good primer that I found interesting.

Pretty much some of the same stuff posted above.
Jan 01, 2011, 01:12 PM
Registered User
Fourdan's Avatar
I have on my HD this table
I don't remember the origin
Jan 01, 2011, 04:42 PM
Registered User
335driver's Avatar
Thanks to all who replied to my questions, makes things a bit clearer.
Jan 01, 2011, 06:16 PM
Registered User

Wake up Astrohog !

We are about ready to close your thread.

Read sooner & faster with us.

Jan 02, 2011, 01:29 AM
Registered User
Well heres how I go about it, very unconventional but will yield the best wing loading and thrust to weight ratios.

IF you believe the model will fly well with .30 nitro find a listed HP for the .30, so I will go with a good OS .32 showing 1 HP

So my next key math, regards the battery pack is the motor and provides the neccessary Horsepower and the motor itself is nothing but a generic gearbox that gets the battery HP into a usable fromat to spin a prop versus heating water.

10g battery weight yields 25 watts usable.

So 1HP = 749 watts so we need a battery that weighs 299grams

Finding a motor frame that can handle the 750watts with least amount of weight is my next key ingrediant after this.

I would go with one of these as the litest grams to watts rated.

Next up is what size prop? considering all the options such as Dia. top speed or flying style, etc, So a ,30 IC is probably 9-10" diam so now its time to run some differant scenarios using online calculator from , props, cell counts and KV to see how it stacks up to 750W.

I use this one:

This looks good.

Check out them specs, 126G motor @ 1 HP, check out that speed and thrust. AS A TWIN thats 10.8 lbs thrust, And this really is conservative for this motor, you can easily prop up with bigger ESC or down for longer flights.

As for a 299g battery capable of the 749watts

This is about 20 minutes research for a nitro conversion.

As a rule for 400Watt and less motors I use

For 400W - 2000W I go Hyperion

Beyond 3000W innrunners on gearboxes from NEU start to look good.

Be interesting to see what you were thinking using the above conventinal means of conversions of guestimating flying weights against X watts per lb??
Latest blog entry: Cappys new Carden Hauler
Jan 02, 2011, 01:43 AM
Registered User
OH best part is as E power we are near or less the same powerplant weight penalty range as Nitro but with calculated power conservatively far exceeding to the prop.

OS 32 9.5oz + muffler= 10.5oz+??
6 oz fuel 6+oz ????

4S battery 10.5oz
motor 4.4oz
Latest blog entry: Cappys new Carden Hauler
Jan 02, 2011, 11:09 AM
Registered User


They get lighter and effectively stronger & faster, as the flight goes on.

We get weaker & slower.

Anybody for a duration race ?
Jan 02, 2011, 07:51 PM
Registered User
Toysrme's Avatar
Be sure you look at what kind of aircraft & what kind of flying it does also.

You have to look at both when the kit was made VS engine thrust at that time period VS what was popular at that time.
.30 sized glow airplane kits have NOT been popular for R/C for decades. .30 sized planes were popular with control-line fliers. They've really only come back due to E-Power and the expense of larger planes!
.40 R/C planes were not particularly popular (outside of trainers, which would fly fine on .25 sized engines LoL!). They are called .40 class engines but in general were used with .45-5X 2-stroke / .60-90 4-stroke sized sport planes.

Say a semi-modern sport kit from the 1990's & up called for a .40 sized 2-stroke glow engine. You would probably be most happy looking at .46 or even .53 (A nice Hot 2-stroke like an irvine .53!)

So for a .40 plane I'd probably go with what Dr Kiwi posted as a .46 engine on a sport plane, or a .53 for a high performance (3D, racing, etc).
Last edited by Toysrme; Jan 02, 2011 at 07:59 PM.

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Selecting the right motor/prop combo? TheFrisco Electric Motor Design and Construction 1 Oct 12, 2010 04:14 PM
Discussion Selecting the right motor/prop combo? TheFrisco Scratchbuilt Indoor and Micro Models 0 Oct 12, 2010 03:45 PM
Question Selecting Right Wire Size PeterH Power Systems 5 Nov 29, 2007 11:25 PM
Discussion need advice on selecting motor noobee Power Systems 4 Feb 26, 2007 08:17 PM
I need advice for the right Outrunner on a new 3D plane... Argyle Power Systems 15 Nov 26, 2005 11:18 AM